In a damaged or faulty power cable of a submarine communications system electrical current can leak to a sea-earth at the position of the fault. Electrolysis of the surrounding sea water destroys the cable whilst producing large quantities of gas. Should gas find a way within the cable and subsequently into the sea casing of an optical repeater or branching unit this can cause serious damage. The establishment of a virtual earth at the position of the fault would limit these effects until a ship could be sent to the site to repair the damaged cable.
In addition to the above, it is sometimes necessary to establish a virtual earth at a branching unit to ensure correct operation of the relays in the presence of a shunt fault and thereby isolate a faulty leg.
Methods of establishing virtual earths on cables of submarine communications systems already exist and one example relies on manual synchronisation between respective operators of two powerfeeds at opposite ends of a submarine cable. The two operators synchronise their actions by telephone such that they both start using an identical power-up rate at the same time. However, to allow for operator error in the synchronisation of the start-up of the ramps only slow power-up rates can be used. Even with a slow power-up rate, the establishment of a virtual earth at a precise location on the cable is heavily dependent on operator performance and as such, prone to human error. Any timing error greater than some critical value will mean that a virtual earth will not be created at the desired location on the cable. Furthermore, this method relies on the stability of a number of unpredictable external parameters, such as the earth""s magnetic field. Magnetic storms, for example, could induce voltage differences in the cables which would affect the stability of the virtual earth.
According to a first aspect of the present invention, a method of establishing a virtual earth at a location in a submarine communications system comprises the steps of, at a powerfeed, detecting an incoming current to the powerfeed generated by a remote powerfeed and automatically modulating an electrical output of the powerfeed in dependence on the detected current to maintain the location of the virtual earth.
The modulation of the powerfeed output is executed automatically and as such a virtual earth can be maintained and established at the desired location on the power cable of the submarine communications system with a substantially reduced risk of error in the location of the virtual earth due to operator error as described above.
Preferably, the electrical output of the powerfeed is controlled in dependence on a series of voltage control values which are calculated in a predetermined manner by sampling the incoming current.
More preferably, the incoming current is sampled to determine a value for the magnitude of the incoming current and the rate of change of the incoming current.
The use of the rate of change of the incoming current to determine the output of the powerfeed enables the virtual earth to be maintained at the desired location on the power cable taking into account dynamic changes in the incoming current.
Preferably, the series of voltage control values are stored in a look-up table and may be output in sequence from the look-up table at a predetermined rate. The look-up table is up-dated periodically to take account of detected changes in the incoming current.
According to a second aspect of the present invention, a powerfeed for a submarine communications system, comprises means for detecting an incoming current to the powerfeed generated by a remote powerfeed, and means for automatically modulating an electrical output of the powerfeed in dependence on the detected current to establish and thereafter maintain the position of a virtual earth on the system.
The powerfeed is arranged such that the modulation of its output is executed automatically and as such a virtual earth can be maintained and established at the desired location on the power cable of the submarine communications system with a substantially reduced risk of error in the location of the virtual earth due to operator error as described above.
Preferably, the means for automatically modulating the electrical output of the powerfeed is arranged to generate a series of voltage control values which are calculated in a predetermined manner by sampling the incoming current.
Preferably, the means for automatically modulating the electrical output of the powerfeed comprises a memory which stores the series of voltage control values in the form of a look-up table, wherein the contents of the look-up table are arranged to be read out sequentially to control the electrical output of the powerfeed.
Preferably, the powerfeed further comprises a voltage output controller and a power converter, wherein the voltage output controller is arranged to receive voltage control values from the look-up table and generate a signal in dependence on this for controlling the output of the power converter.
Preferably, the powerfeed further comprises a voltage sensor for sensing the voltage appearing at an output of the powerfeed, the voltage sensor being arranged to feed back a control signal to the voltage output controller in dependence on this, wherein the voltage output controller is arranged to compare the output of the voltage sensor and the output of the look-up table to generate an error signal, the error signal being fed to control the voltage output of the power converter.
The use of an error signal to control the output of the powerfeed ensures that the output is contolled accurately and as such the virtual earth will be maintained in the desired location.
Preferably, the means for automatically modulating an electrical output of the powerfeed comprises a digital signal processor which may be programmed to up-date periodically the contents of the look-up table in dependence on the incoming current and the digital signal processor may comprise a memory storing computer executable instructions for performing the method according to the first aspect of the present invention.
According to a third aspect of the present invention a submarine communications system comprises a first powerfeed connected to a first end of a submarine power cable and a second powerfeed connected to a second end of the submarine power cable in which at least one of the powerfeeds is a powerfeed according to the first aspect of the present invention.